1. Field of the Invention
The invention concerns a control mechanism for a coupling device such as, for example, a clutch, a variable speed drive or a brake, and is particularly concerned with a motorized mechanism of this kind in which a linking mechanism generally comprising gearing and a toothed sector is disposed between the motor and means for actuating the coupling device, comprising a level mounted to pivot about a shaft, for example.
The invention is more particularly directed to an improvement made to this type of device in order to improve its "irreversibility", that is to say the ability to control with precision the displacement of the actuator lever, whatever the actuation direction, avoiding in particular any "backlash" of the motor after each period of actuation as a result of reaction forces transmitted to the linking mechanism by the coupling device itself.
2. Description of the Prior Art
British patent application No. 2 117 076 describes several embodiments of a motorized control mechanism of this kind suitable for actuating a coupling device of the type defined hereinabove. Specifically, a system, of this kind has been developed for the motorized control of an automobile vehicle clutch in order to eliminate the traditional clutch pedal, but it may equally well serve to control a variable speed drive (especially of the belt type with variable geometry pulleys) or even a brake.
In one embodiment described in the above-mentioned document the drive shaft of the motor comprises a portion forming a lead screw meshing with a gear itself interlocking with a toothed sector attached to an actuator lever. This is fastened to a forked lever which, on pivoting, cooperates with an abutment member (a clutch release bearing, for example) movable along an axis and adapted to actuate a diaphragm-shaped coupling spring. An elastic device for moderating the operating force is associated with the toothed sector and/or the gear to reduce the force required of the electric motor over the coupling or decoupling travel, which makes it possible to employ an electric motor of relatively low power, comparable with that of an automobile windshield wiper motor in the case of a standard clutch, for example.
In one particularly advantageous embodiment, the elastic force moderator device is a simple spring coupled to a special gearwheel of said gearing.
This type of device can function satisfactorily provided that "irreversible" transmission between the motor and the coupling device is achieved. Such "irreversibility" may be provided at the level of the mechanical coupling between the lead screw and the gearing, by choosing the inclination of the threads appropriately. The problem becomes particularly severe, however, when the motor is fed a series of electrical pulses, where each pulse is intended to produce a predetermined displacement of the movable abutment member. If the "irreversibility" of the mechanism is less than perfect, backlash is observed in the gearing system after each pulse, caused by the reaction of the coupling device itself. The cumulative effect of such backlash may very seriously compromise the precision with which the movable abutment member is positioned, as a consequence of which control becomes unreliable or even impossible. It has also been found that the "irreversibility" achieved at the level of the lead screw and the gearing is difficult to reproduce in an industrial context, given a deliberately limited motive power. An object of the invention is to resolve this problem.